Digital light projection (DLP) technology has been increasingly used for optical display systems, such as those found in upscale home theater systems. This technology has begun replacing traditional cathode ray tube technology, since it can provide high image quality, without the bulk and power requirements associated with the older technology.
In essence, a DLP system is comprised of a digital micromirror device (DMD), made up of an array of thousands or even millions of bistable mirror elements, interacting with a light source and a projection surface. Each of the mirror elements of the DMD may switch between two positions, corresponding to an open or closed light configuration, based on the angle at which the mirror tilts towards the light source. A micromirror is in an open position when it is oriented to reflect the light source onto the projection surface. A micromirror is in a closed position when it is oriented so that none of the light provided by the light source is projected onto the projection surface. Thus, each micromirror can be oriented in either an open or “on” position, or a closed or “off” position.
By rapidly turning a particular micromirror “on” and “off”, the appropriate shade of light can be projected for a particular pixel on the projection surface. And color hues may also be added to a DMD projection system by time multiplexing of the white light source through a color wheel. In practice, the micromirrors alternate between open and closed positions so fast that the human eye cannot discern the discreet “on” and “off” positions of each micromirror. Instead, the human eye extrapolates the discreet binary images projected by each mirror element into a wide variety of pixel shades and hues. In this way, DMDs allow for the accurate reproduction of the whole array of necessary shades and hues by taking advantage of the human eye's averaging of quickly varying brightnesses and colors.
Typically, each micromirror is oriented in either the open or closed position using electrostatic forces generated by corresponding electrodes. Each micromirror is located atop a hinge mechanism, and an electrode is located on either side of the hinge. These electrodes are typically formed on a semiconductor substrate beneath the micromirrors. Whenever an appropriate voltage is applied to an electrode, it creates an electrostatic force capable of pivoting the micromirror on its hinge. Only one of the two electrodes will be active at any specific moment in time, corresponding to either the open or closed position. By way of example, if a sufficient voltage is applied to the first electrode, then its micromirror would be pulled out of its neutral alignment, so that it angles towards the light source and will reflect light onto the projection surface. This would correspond to an open or “on” position for the micromirror. If a sufficient voltage is applied to the second electrode for the same micromirror (while there is no voltage applied to the first electrode), then the micromirror would pivot to angle away from the light source. In this closed or “off” position, no light would be reflected upon the projection surface. So, each micromirror pivots between open and closed positions based on the electrostatic forces applied on the mirror by the electrodes on either side of the pivot point.
This conventional DMD approach generally works well, allowing for quite accurate and crisp image reproduction quality. Nevertheless, DMD image quality has historically been impacted by stiction, which is a tendency for each micromirror element to stick when in contact with the electrode contact surface (against which the electrostatic force holds the micromirror in either the open or closed position). This stiction is associated with Van der Waal's forces, surface contamination, and surface friction, and can cause a delay in the movement of the micromirrors, resulting in possible image degradation.
To overcome this stiction problem, conventional DMDs apply large voltages to the electrodes and the micromirrors in sequence, essentially pulsing the electrodes and the micromirrors to reorient the micromirrors into their neutral starting position. This type of reset pulse breaks the stiction, and allows the micromirrors to move freely from one orientation to another. But the use of high-voltage pulses to overcome stiction acts as a limiting factor concerning the size and expense of DMDs, since the transistors operating the electrodes must be capable of handling the high voltage needed to overcome stiction. Likewise, the use of high voltages requires a sufficient gap between each of the micromirrors in order to prevent micromirrors with different voltages from being mutually attracted, and this gap requirement acts as a limitation on the contrast available for the DMD. So, problems associated with the present high-voltage pulse technique for overcoming stiction have led to investigations into alternative techniques for addressing stiction problems.